r13739: a fairly major overhaul of the opendb code to allow the BASE-DELETE

r13739: a fairly major overhaul of the opendb code to allow the BASE-DELETE
test to pass. To try to make the code a bit more understandable, I
moved to using an IDL description of the opendb tdb record format.

One of the larger changes was to make directory opens and creates go
via the opendb code, so directory operations now obey all the share
mode restrictions, as well as delete on close semantics. I also
changed the period over which the opendb locks are held, to try to
minimise races due to two open operations happening at the same time.
(This used to be commit cd2602d05725e1734b0862131dd91601c6b6d51a)